Ecophysiological responses of Jussiaea rapens to cadmium exposure

Shoots of Jussiaea rapens Linn were exposed to nutrient solutions containing 0, 7, 15, 30, 60 mg L⁻¹ cadmium (Cd²⁺) and the effects on plant physiology investigated after treatment for 2 and 5 days. The net photosynthesis rate was inhibited by 54.8–62.7%, but only at the two highest [Cd²⁺]. Stomatal conductance showed an initial decline from 0.01 to 0.07 mol m⁻² s⁻¹ after a 2-d exposure to high [Cd²⁺] but recovered after a 5-d exposure. The Chl fluorescence parameters F_v/F_m, qP and ETR also declined by 16, 87, and 90%, respectively, after 5-d at the highest [Cd²⁺], which is consistent with damage to the activity of Photosystem II. Chlorophyll a (Chl a), and b and total chlorophyll (Chl) all exhibited content reductions, though in the case of Chl b it required a 5-d exposure. Significant increases in superoxide dismutase (SOD) activity were observed at all [Cd²⁺], the greatest increase being 149%. Likewise, carotenoid content increased by five-fold relative to the untreated control values. The increase in SOD activity and carotenoid suggests that one effect of the Cd²⁺ caused oxidative stress in J. rapens. Root vitality was also negatively influenced by all Cd²⁺ treatments. However, the [Cd²⁺] values inhibiting the photosynthetic parameters were greater than those reported for several other species, which suggests that J. rapens may have some degree of tolerance to this toxic metal.